In many videocoding algorithms such as H.261, H.263, MPEG-1, MPEG-2, and MPEG-4, hybrid coding methods are used to efficiently represent video sequences. See, for example Draft of MPEG-2: Test Model 5, ISO/IEC JTC1/SC29/WG11, April 1993; and Draft ITU-T Recommendation H.263, ITU-T SG XV, December 1995. In these schemes, motion compensated prediction (MCP) is first performed to reduce the temporal redundancy in a video sequence and the resultant MCP errors are then encoded with discrete cosine transform (DCT), quantization, and entropy coding. In such hybrid coding systems, there is always a trade-off in distortion (D) and rate (R) between MCP and residual coding (RC). If a coder assigns too many bits for motion vector encoding, it may find itself with not enough bits left to properly encode MCP errors, and vice versa. It is therefore important to find an optimal bit allocation between MCP and RC that provides the smallest overall distortion.
A rate-distortion optimized coding mode selection scheme proposed in Wiegand, T, and M. Lightstone, D. Mukherjee, T. G. Campbell and S. K. Mitra, "Rate-distortion optimized mode selection for very low bit rate video coding and the emerging H.263 standard," IEEE Trans. Circuits Syst. Video Tech., vol. 6, no. 2, pp. 182-190, April, 1996, provides improvement in rate control as well as in picture quality, especially in very low bitrate video coding applications. However this scheme performs Discrete Cosine Transforms (DCT), quantization, de-quantization, and entropy-encoding to obtain distortion (D) and rate (R) for residual coding (RC). This required high level of computational complexity proves impractical for real-time implementations.